After a lot of hunting around for a 3GP command line converter, finally tracked down a windows build of the mighty ffmpeg ( normally built for linux ). The trick with ffmpeg is to get a version that has the specifc audio convertor for 3GP ( the file format used by most mobile phones )

Because I am nice, I've included a ZIP file containing all the things you will need.

The following is a simple command line to encode near any type of video to 3GP:

( Replace source,temp and dest with relevent filenames ... dest MUST have a 3GP extension )